Modern society requires that enormous amounts of information be transmitted between users in a relatively error-free manner. Most of the information is communicated as digital information, primarily because digital techniques allow more information to be communicated quickly and reliably, and because a significant amount of the information is transferred between computers. The use of computers and the evolution of computer technology is responsible for much of the increased demand for information communication. The demand for information communication has increased dramatically during the past few years and is expected to continue well into the future.
The typical medium which carries a significant amount of the information is electrical conductors or copper wires. The telephone system, having been installed for many years, is the primary media used for local or localized communications. Using wired media for telecommunications and high speed data communications creates difficulties, and these difficulties arise because of the wires. Electrical wires introduce a practical limit to the physical length or distance over which the information can travel. Lengthy conductors attenuate the signals to the point where the recognition of signals becomes difficult or impossible. Signals conducted over the wires also have a finite limit to the signaling frequency and hence the amount of information which they can carry. Furthermore, noise is relatively easily picked up or induced into the wires, and the noise tends to corrupt the signals carried by the wires. Wire conductor media is also difficult or impossible to install in many situations. Some metropolitan areas simply have no available space to accommodate the additional conductors within utility conduits, and gaining access to buildings and right-of-way to install the conductors is usually difficult or impossible and is certainly costly. For these and other reasons, many of the advancements in communications have focused on wireless media for communicating information.
Radio frequency (RF) transmissions avoid many of the physical problems associated with wired media. The atmosphere becomes the medium for the RF communications, and thus physical limitations associated with access, space, and right-of-way are no longer paramount problems. However, because the atmosphere is freely available for use by all authorized users, the possibility of interference is always present. Various techniques have been devised to minimize RF interference, but those techniques are relatively expensive to implement. Furthermore, even those techniques are not effective to assure that enormous amounts of information can be communicated reliably through RF broadcasts, simply because the information is broadcast and can not be confined to secure communication channels or links which could eliminate sources of interference.
Optical media offers many advantages compared to wired and RF media. Large amounts of information can be encoded into optical signals, and the optical signals are not subject to many of the interference and noise problems that adversely influence wired electrical communications and RF broadcasts. Furthermore, optical techniques are theoretically capable of encoding up to three orders of magnitude more information than can be practically encoded onto wired electrical or broadcast RF communications, thus offering the advantage of carrying much more information.
Fiber optics are the most prevalent type of conductors used to carry optical signals. Although the disadvantage of fiber optic conductors is that they must be physically installed, the fact that an enormous amount of information can be transmitted over the fiber optic conductors reduces the number of fiber optic conductors which must be installed. This avoids some of the problems in metropolitan areas were space for additional cables is difficult to obtain. In those circumstances where the information is communicated over long distances, fiber optic conductors are the typical medium employed for such long-haul transmissions.
Free-space atmospheric links have also been employed to communicate information optically. A free-space link extends in a line of sight path between the optical transmitter and the optical receiver. Free-space optical links have the advantage of not requiring a physical installation of conductors. Free-space optical links also offer the advantage of selectivity in eliminating sources of interference, because the optical links can be focused directly between the optical transmitters and receivers, unlike RF communications which are broadcast without directionality. Therefore, any adverse influences not present in this direct, line-of-sight path or link will not interfere with optical signals communicated.
Despite their advantages, optical free-space links present problems. The quality and power of the optical signal transmitted depends significantly on the atmospheric conditions existing between the optical transmitter and optical receiver at the ends of the link. Rain drops, fog, snow, smoke, dust or the like in the atmosphere will refract or diffuse the optical beam, causing a reduction or attenuation in the optical power at the receiver. The length of the free-space optical link also influences the amount of power attenuation, because longer free-space links will naturally contain more atmospheric factors to potentially diffuse the optical beam than shorter links. Furthermore, optical beams naturally diverge as they travel greater distances. The resulting beam divergence reduces the amount of power available for detection. If the attenuation of the optical beam is sufficiently great, the ability to recognize the information communicated on a reliable basis is diminished, and the possibility that errors in communication will arise is elevated. Atmospheric attenuation particularly diminishes the probabilities of error-free communications at higher transmission frequencies, because atmospheric attenuation naturally occurs to a greater extent at higher optical frequencies, i.e. shorter wavelengths, than at lower optical frequencies.
One approach to reducing the adverse influences of atmospheric attenuation is to use laser beam transmissions in the free-space links at frequencies which are capable of greater penetration and less refraction or diffusion by atmospheric influences. Unfortunately, the more penetrating frequencies are sometimes also the ones which can easily damage human eyes. To maintain safety while still avoiding some of the problems from atmospheric attenuation, the amount of power which is optically transmitted at these more penetrating frequencies is substantially limited. Since the more penetrating frequencies are also subject to beam divergence, reducing the power still complicates the reliable communication of information. Consequently, the reduced power transmission levels counter-balance the benefits of the lesser atmospheric attenuation at the more penetrating frequencies. Because of the reduced power at the more penetrating frequencies, the effective length of the free-space optical links is still limited.
Furthermore, the more-penetrating, free-space optical frequencies are different from those frequencies which are typically employed to transmit information over long-haul fiber communication systems. An electro-optical conversion is required to convert the fiber link backbone transmission frequency to the free-space transmission frequency. An electro-optical conversion involves converting the higher frequency optical signals to electrical signals and back to optical signals at the more penetrating laser frequency, and vice versa. Additional equipment is required to accomplish the conversion, resulting in an increase in the cost and complexity of the terrestrial optical communications network.
In addition, electro-optical conversions also introduce the possibility that errors will be created during the conversion, particularly under the common situation of the fiber optic signal carrying information at multiple different wavelengths. Common optical detectors respond to information in a broad frequency range or wavelength band, and this broad-band response destroys the information carried at specific wavelengths. To avoid this problem and to maintain the information present in the different, specific wavelength optical signals, the optical signal must first be filtered into its different wavelength components. Thereafter each different wavelength component must be separately electro-optically converted, and then all of the separate converted components combined back into a single optical signal. The complexity of this process raises the possibilities of introducing errors in the information communicated and increases the costs of the equipment used in the terrestrial optical communication network.
Electro-optical conversion has also been used to amplify the light signals conducted over fiber optic cables. The light signals conducted over fiber cables will attenuate, and it is periodically necessary to amplify those signals in order to maintain signal strength. Recently however, erbium doped fiber amplifiers (ERDAs) have been developed to amplify the light signals optically, without requiring electro-optical conversion, as the light signals pass through the optical fiber. ERDAs allow light to be amplified in a relatively wide wavelength band (about 30 nanometers (nm)) around a 1.55 micrometer (um) fundamental wavelength. ERDAs are of particular advantage in long haul telecommunications systems, because these systems normally operate in the 1.55 um wavelength range. The broad band amplification of ERDAs around the 1.55 um fundamental frequency allows the ERDAs to be integrated into systems using wavelength division multiplexing (WDM), resulting in the ability to communicate separate information at different wavelengths simultaneously in the same fiber. Thus, ERDAs are of particular importance and value in long haul fiber telecommunication systems because electro-optical conversions can be avoided.
It is with respect to these and other background information factors relevant to the field of terrestrial optical communications that the present invention has evolved.